A friend of mine who taught middle school called it “hormone poisoning.” But National Geographic takes a slightly more serious look at why adolescents do the things they do:
My son’s high-speed adventure raised the question long asked by people who have pondered the class of humans we call teenagers: What on Earth was he doing? Parents often phrase this question more colorfully. Scientists put it more coolly. They ask, What can explain this behavior? But even that is just another way of wondering, What is wrong with these kids? Why do they act this way? The question passes judgment even as it inquires.
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Imaging work done since the 1990s shows that these physical changes move in a slow wave from the brain’s rear to its front, from areas close to the brain stem that look after older and more behaviorally basic functions, such as vision, movement, and fundamental processing, to the evolutionarily newer and more complicated thinking areas up front. The corpus callosum, which connects the brain’s left and right hemispheres and carries traffic essential to many advanced brain functions, steadily thickens. Stronger links also develop between the hippocampus, a sort of memory directory, and frontal areas that set goals and weigh different agendas; as a result, we get better at integrating memory and experience into our decisions. At the same time, the frontal areas develop greater speed and richer connections, allowing us to generate and weigh far more variables and agendas than before.
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As Laurence Steinberg, a developmental psychologist specializing in adolescence at Temple University, points out, even 14- to 17-year-olds—the biggest risk takers—use the same basic cognitive strategies that adults do, and they usually reason their way through problems just as well as adults. Contrary to popular belief, they also fully recognize they’re mortal. And, like adults, says Steinberg, “teens actually overestimate risk.”
So if teens think as well as adults do and recognize risk just as well, why do they take more chances? …[B]ecause they weigh risk versus reward differently: In situations where risk can get them something they want, they value the reward more heavily than adults do.
The whole article covers more than just this snip, and actually does a pretty good job of answering the question: “Why, for the love of Pete, why?”